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The growing problem wih weeds that have become resistant to the most common herbicide used by American corn, soybean and cotton growers has gotten so serious that new strategies are needed to combat them.

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When Penn State weed scientist David Mortensen told members of the U.S. House Oversight Committee this summer that the government should restrict the use of herbicide-tolerant crops and impose a tax on biotech seeds to fund research and educational programs for farmers, it caused quite a stir.

The growing problem with weeds that have become resistant to the most common herbicide used by American corn, soybean and cotton farmers has gotten so serious that new strategies are needed to combat them, he contended.

Mortensen should know. The professor of weed ecology in the College of Agricultural Sciences has spent his career researching weeds that affect agricultural production, sustainable ways to control them, and the relationships between crops, native and invasive weeds, and pollinators. He has published several peer-reviewed papers on the subject in recent years.

The resistant weeds cannot be killed by the sole use of glyphosate, the active ingredient in Roundup herbicide. Roundup has become broadly popular with farmers since the advent more than a decade ago of soybeans, cotton, corn and other crops that are resistant to the chemical. The weeds now infest about 11 million acres — a fivefold increase in three years, Mortensen reported.

The problem is most prevalent in cotton and soybean fields in the South, but is spreading to other regions. And he told lawmakers it will get worse if farmers don't take measures to control the weeds, including non-chemical methods such as planting cover crops to suppress weeds, rotating crops and spraying herbicides other than glyphosate.

"With the rise of glyphosate-resistant weeds, farmers have to quit relying so heavily on Roundup to control weeds," he said. "Farmers value the convenience and simplicity of these crops without appreciating the long-term ecological and economic risks."

Testifying before the Domestic Policy Subcommittee of the House Oversight and Government Reform Committee July 28, Mortensen explained that weed management is a serious matter for farmers. While weed management almost always comprises several tactics, herbicide use is central and accounts for 70 percent of all pesticides used in agriculture.

"Since the mid-1990s, adoption of genetically engineered crops resistant to the herbicide glyphosate has been widespread, and herbicide-resistant crops are now grown on more than 143 million acres of cropland internationally, with 92 percent of the U.S. soybean crop planted to glyphosate-resistant varieties," he said.